Fe-Fe bonding in the rhombic Fe4 cores of the Zintl clusters [Fe4E18]4- (E = Sn and Pb).

We report here the synthesis and characterization of two endohedral Zintl-ion clusters, [Fe4Sn18]4- and [Fe4Pb18]4-, which contain rhombic Fe4 cores. The Fe-Fe bond lengths are all below 2.5 Å, distinctly shorter than in the corresponding Cu clusters, indicating the presence of Fe-Fe bonding. Subtle differences in the structure of the Fe4 core between the two clusters suggest that the change in tetrel element causes a change in electronic ground state, with a very short Fe-Fe bond length of 2.328 Å present across the diagonal of the rhombus in the lead case.